The MIMO technology, which is a technology in which a transmitter/receiver uses multiple antennas to transmit/receive information, is a research hotspot of the multiple antenna technology in the field of wireless mobile antenna and is also one of key technologies which must be used in the next generation mobile communication system. This technology uses multiple paths to fight back various random fading in channels, effectively avoiding co-channel interference and improving channel quality, thus improving network reliability and communication service quality. By using space resources, theoretically, system capacity and spectral efficiency may be multiplied several times without consuming additional air interface resources (time, frequency).
The MIMO technology mainly includes two applications: diversity Space Time Coding (STC) and Spatial Multiplexing (SM), where the diversity STC includes transmit diversity and receive diversity.
The diversity STC technology takes advantage of time and space simultaneously and enhances diversity gain and coding gain without increasing system capacity, its principle is shown as FIG. 1. It can be seen from FIG. 1 that input characters, that is, information sources, are divided into two groups, each group comprising two characters. In the first character time, two characters [C1, C2] in each group are transmitted from two antennas simultaneously, and in the next character time, the two characters are transformed into [−C2*, C1*] and transmitted from the two antennas again. Therefore, a receiving antenna can receive two different forms of two characters within the two character times. Compared with the case where only one form of two characters are received in the two character times, the correctness rate of characters restored using the decoding technology is greatly improved, it is shown that bit error rate is lowered and link reliability is increased, further increasing signal coverage area. When the coverage area is invariant and the user requirement of the bit error rate is constant, the diversity gain may be turned into increase of data transmission rate, for example, using a higher modulation and coding mode.
The SM technology uses spaces, as shown in FIG. 2. A high-speed data stream is divided into parallel data streams for simultaneous transmission, and at this point the transmitted data of each antenna are different, and are spatially demodulated and multiplexed and re-combined into a high-speed serial data stream at a receiving end. Using this method, transmission rate and throughput of the system can be greatly increased.
Since the wireless channel changes in real time, at some moments, the diversity STC can be used to get a better channel gain and improve the reliability of link transmission; while at other moments, the SM technology can be used to improve the channel transmission rate, thereby improving channel throughput. Using either the STC technology or the SM technology cannot maximally use the limited frequency band resources.
The Adaptive Modulation and Coding (AMC) is a selectively link adaptation method in the wireless communication technology. The AMC provides mobility to cooperate with the modulation and coding mode to make each user achieve an average channel condition. The downlink AMC is usually implemented in the case where the non-MIMO mode or the MIMO mode is the STC mode. The AMC alone may also be used to improve the system throughput. When both the AMC and the MIMO are used, if the AMC is adjusted separately, the case where the data transmission is unstable or the throughput cannot achieve an optimal condition may occur.
A MIMO mode fed back by a terminal means that a terminal estimates downlink channel quality based on downlink information, chooses a MIMO mode which is believed to be suitable, and feeds this MIMO mode information back to a base station via an uplink. The base station may select a MIMO mode to be used based on the MIMO mode proposed by the terminal. In other words, the MIMO mode fed back by the terminal can be used as a basis to determine channel conditions.
Switching of the downlink AMC and that of the MIMO in existing systems are carried out separately, adjustment of one of the downlink AMC and the MIMO do not consider whether the other is adjusted and whether the adjustment has effect on system stability. Facing the same channel condition (for example, a CINR (Carrier to Interference plus Noise Ratio) value is invariant), there are only two choices when the AMC and the MIMO are adjusted separately: the STC mode and the SM mode under the current DIUC (Downlink Interval Usage Code) value are used. When the STC mode under the current DIUC is used, link reliability is guaranteed, but traffic cannot be increased; when the SM mode under the current DIUC is used, the traffic is theoretically increased, but the link reliability cannot be guaranteed.